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How to correctly interconnect multiple batteries to form one larger bank.


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Paul

How to correctly interconnect multiple batteries to form one larger bank.

Reprinted by written permission from David Small.

www.smartguage.co.uk


Two things I have noticed in my (more than) 20 years in this business are that:-

A. Many "specialists" simply tell you..... "do it this way, this is the correct way" without ever showing why they consider it to be the correct way, and often it isn't, which is perhaps why they couldn't show you why it is(!)

B. Some things have been done for so long, in a certain manner, that it seems they must be the best way of doing it. Otherwise why hasn't another method appeared?

Here at SmartGauge Electronics we always show you why one method is better. We don't expect you to take our word for it. We will happily use practical examples, theory, maths or whatever else it takes to show the results of various ways of doing things.

Interconnecting multiple batteries to form one larger bank is one case in point. Though in this case, newer methods have emerged over the years. Unfortunatley they still aren't perfect.
Here is a diagram showing the old way of interconnecting 4 batteries to form one larger bank. This is a method that we still see in many installations.

batt_old.gif

Method 1

Notice that the connections to the main installation are all taken from one end, i.e. from the end battery.

The interconnecting leads will have some resistance. It will be low, but it still exists, and at the level of charge and discharge currents we see in these installations, the resistance will be significant in that it will have a measurable effect.

Typically the batteries are linked together with 35mm cable in a good installation (often much smaller in a poor installation). 35mm copper cable has a resistance of around 0.0006 Ohms per metre so the 20cm length between each battery will have a resistance of 0.00012 Ohms. This, admittedly, is close to nothing. But add onto this the 0.0002 Ohms for each connection interface (i.e. cable to crimp, crimp to battery post etc) and we find that the resistance between each battery post is around 0.0015 Ohms.

If we draw 100 amps from this battery bank we will effectively be drawing 25 amps from each battery. Or so we think.

In actual fact what we find is that more current is drawn from the bottom battery, with the current draw getting progressively less as we get towards the top of the diagram.

The effect is greater than would be expected.

Whilst this diagram looks simple, the calculation is incredibly difficult to do completely because the internal resistance of the batteries affects the outcome so much.

However look at where the load would be connected. It is clear that the power coming from the bottom battery only has to travel through the main connection leads. The power from the next battery up has to travel through the same main connection leads but in addition also has to travel through the 2 interconnecting leads to the next battery. The next battery up has to go through 4 sets of interconnecting leads. The top one has to go through 6 sets of interconnecting leads. So the top battery will be providing much less current than the bottom battery.

During charging exactly the same thing happens, the bottom battery gets charged with a higher current than the top battery.

The result is that the bottom battery is worked harder, discharged harder, charged harder. It fails earlier. The batteries are not being treated equally.

Now in all fairness, many people say "but the difference is negligible, the resistances are so small, so the effect will also be small".

The problem is that in very low resistance circuits (as we have here) huge differences in current can be produced by tiny variations in battery voltage. I'm not going to produce the calculations here because they really are quite horrific. I actually used a PC based simulator to produce these results because it is simply too time consuming to do them by hand.

Battery internal resistance = 0.02 Ohms
Interconnecting lead resistance = 0.0015 Ohms per link
Total load on batteries = 100 amps
The bottom battery provides 35.9 amps of this.
The next battery up provides 26.2 amps.
The next battery up provides 20.4 amps.
The top battery provides 17.8 amps.

So the bottom battery provides over twice the current of the top battery.

This is an enormous imbalance between the batteries. The bottom battery is being worked over twice as hard as the top battery. The effects of this are rather complex and do not mean that the life of the bottom battery will be half that of the top battery, because as the bottom battery loses capacity quicker (due to it being worked harder) the other three batteries will start to take more of the load. But the nett effect is that the battery bank, as a whole, ages much quicker than with proper balancing.

I have to be honest now and say that when I first did this calculation in about 1990 I completely refused to believe the results. The results seemed so exaggerated. So much so that I wired up a battery bank and did the experiment for real, taking real measurements. The calculations were indeed correct.

batt_new.gif

Method 2

All that has changed in this diagram is that the main feeds to the rest of the installation are now taken from diagonally opposite posts.

It is simple to achieve but the difference in the results are truly astounding for such a simple modification.
The connecting leads, in fact, everything else in the installation remains identical.

Also, it doesn't matter which lead (positive or negative) is moved, Whichever is easiest is the correct one to move.

The results of this modification, when compared to the original diagram are shown below. Only that one single connection has been moved.

After this simple modification, with the same 100 amp load....
The bottom battery provides 26.7 amps of this.
The next battery up provides 23.2 amps.
The next battery up provides 23.2 amps.
The top battery provides 26.7 amps.

This is quite clearly a massive improvement over the first method. The batteries are much closer to being correctly balanced. However they are still not perfectly balanced.

How far is it necessary to go to get the matching equal?

Well, the better the quality of the batteries, the more important it becomes. The lower the internal resistance of the batteries, the more important it is to get them properly balanced.

So that now leaves the question of whether or not there is a wiring method to perfectly balance the batteries.

Before getting to that, it should be pointed out that doing the calculation is not actually required in order to arrive at the ultimate interconnection method. I simply did them to show the magnitude of the problem.

In order to get a better balancing it is simply necessary to get the number of interconnecting links as close as equal between each battery and the final loads.

In the first example the power from the bottom battery passed through no interconnecting links. The top battery passed through 6 links.

In the 2nd example (the much improved one), the power from the top and bottom battery both passed through a total of 3 links. That from the middle 2 batteries also both passed through 3 links which begs the question "why were they not therefore perfectly balanced?". The answer is that some of the links have to pass more total current and this therefore increases the voltage drop along their length.

And now we get to the correctly wired version where all the batteries are perfectly balanced.


 

 

All images and text content Copyright 2005, 2013 - SmartGauge Electronics.

Reprinted by written permission from David Small.

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Paul

How to correctly interconnect multiple batteries to form one larger bank.

Reprinted by written permission from David Small.

www.smartguage.co.uk

batt_v_new.gif
Method 3

This looks more complicated.

It is actually quite simple to achieve but requires two extra interconnectng links and two terminal posts.

Note that it is important that all 4 links on each side are the same length otherwise one of the main benefits (that of equal resistance between each battery and the loads) is lost.

The difference in results between this and the 2nd example are much smaller than the differences between the 1st and 2nd (which are enormous) but with expensive batteries it might be worth the additional work. Most people (myself included) don't consider the expense and time to be worthwhile unless expensive batteries are being fitted or if the number of batteries exceeds 8.

This method isn't always so easy to install because of the required terminal posts. In some installations there is simply no room to fit these. So, thanks to a colleague, we can also present another wiring method that achieves perfect battery balancing...............


batt_new_2.gif

Method 4

And here it is.

This looks odd but it's actually quite simple. What has been done here is to start with 2 pairs of batteries. Each wired in the proper "cross diagonal" method. Then each pair is wired together, again in the cross diagonal method.

Notice that for each individual battery, the current always goes through a total of one long link and one short link before reaching the loads.

This method also achieves perfect balance between all 4 batteries and may be easier to wire up in some installations. Many thanks to "smileypete" from www.canalworld.net/forums for this idea.

 

-------------------------------------------------------

 

There really is no excuse whatsoever (apart from, perhaps, incompetence or laziness) for using the first example given at the top of this page.

The other three methods achieve much better balancing with the final two achieving perfect balancing between all four batteries.

I think I am right in saying that this is the only example I have ever come across where doing something the correct way actually looks less elegant than doing it incorrectly.

Finally, if you only have 2 batteries, then simply linking them together and taking the main feeds from diagonally opposite corners cannot be improved upon.

Once the number of batteries gets to 3 or more then these other methods have to be looked at.

With a large number of batteries it may be necessary to go to the 3rd method shown above.

Even with 8 batteries it is possible to get reasonable balancing by placing the main "take off" feeds from somewhere down the chain instead of from the end batteries. Remember, count the number of links each battery needs to run through to reach the final loads and get these as equal as possible.

Finally, if your battery bank has various take off points on different batteries, change it now! It is extremely bad practice. Not only does it mess up the battery balancing, it also makes trouble shooting very much more complicated and looks awful.

And finally, finally, we keep getting asked where the chargers should be connected to. We didn't address this question because it seemed so blatantly obvious where they should be connected that it never occurred to us that anyone might be unsure. The chargers should always be connected to the same points as the loads. Without exception.

 

 

All images and text content Copyright 2005, 2013 - SmartGauge Electronics.

Reprinted by written permission from David Small.

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thebob

Nice post. I would try to implement method 3 using a solid copper busbar. Although Method 4 appears balanced, there is a difference between internal battery resistance depending on if the battery is being charged or discharged! I feel that dendrite formation will be accelerated in excess of those that by natural self-discharge.

 

I'm a great believer that solar installations should also incorporate a wind turbine, to help offset the 12 hour charge/discharge cycle as wind also blows at night. Although they are a little noisy. A large ballast can be used to dump excess turbine power into a hot water tank during the day.

 

Charge controller selection and positioning, is also critical, it isn't an area to save money. What are you building Paul, give us some details.

 

I designed several projects in my mind around that large Italian alternator that you had at one time.

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SkyMan

 

I would try to implement method 3 using a solid copper busbar.

I thought of that too but the links to the connection point need to be the same.  However, this would be a substantial improvement over method 1 and quit a bit simpler than 3 or 4 which would require special lugs and more cable.
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Jsteam

Interesting post, however I saw nothing about the problems with connecting batteries in parallel, ie; the losses caused by circulating currents, whether the batteries are charging or discharging! Not even identical batteries will be exactly the same in terms of their characteristics! The problem can be prevented by using  only series connection of the cells, this of course means sizing and buying suitable amp-hour capacity batteries to avoid the need for paralleling! I know sometimes we have to make do with what we can get at the time in which case Davids article above will be of use.

:drink:  :drink:

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Headshot

Interesting topic. Connecting positive to negative, positive to negative, positive to negative (series) gives you three times the voltage without changing the amperage. You can add as many as you need to get the desired voltage. Connecting positive to positive to positive and negative to negative to negative (parallel) gives you three times the amperage without changing the voltage. You can hook as many together as you need to give you the desired amperage. In the large battery backup systems I have seen (we had them all over the place out on the Utah Test and Training Range), they hooked the batteries up in series to get the desired voltage, and then paralleled banks of batteries (connected in series) if they needed more amperage.

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Paul

What are you building Paul, give us some details.

 

As you may know, for a long time, I have been doing extensive research on alternative energy, and keeping constant voltage supplied to a home. Originally, I was going to go with a solar array, complete with batteries, inverters, etc., in the Philippines. However, things changed when I left the Philippine sofr the states, and ultimately ended up here - Cambodia.

 

recently, I have been looking in a different direction. I have been looking at voltage inverters with chargers. With one of those, I would run deep cycle batteries in a series-parallel configuration.

 

 

I designed several projects in my mind around that large Italian alternator that you had at one time.

 

Bob, you would have to PM "HeyMike", as he should have that alternator. I doubt he has a use for it. If he does not, and if you want it, it is yours. I also have a 3,000 watt voltage inverter (output for US voltage, though), either there, or at Vivian's home in Barili, if interested.

 

--------------

 

Recently, I finally started organizing a folder (with sub folders) on Drop Box. It has researched files, information, and links, that I have been collecting for some time. If anyone would like for me to share these files and information with you, please send your Drop Box email address to me via PM, email, SMS, carrier pigeon, smoke signals, or any other way you can get it to me, and I will gladly share my files with you.

Edited by Paul
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Paul

Interesting post, however I saw nothing about the problems with connecting batteries in parallel, ie; the losses caused by circulating currents, whether the batteries are charging or discharging! Not even identical batteries will be exactly the same in terms of their characteristics! The problem can be prevented by using  only series connection of the cells, this of course means sizing and buying suitable amp-hour capacity batteries to avoid the need for paralleling! I know sometimes we have to make do with what we can get at the time in which case Davids article above will be of use.

 

I am in hopes that, by wiring my batteries (starting off with four 130ah deep cycle batteries) in series-parallel, I have no issues.

 

I want to go with higher voltage to the inverter, thus allowing for a lower ampere draw on the bank.

Edited by Paul
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Paul


I'm a great believer that solar installations should also incorporate a wind turbine, to help offset the 12 hour charge/discharge cycle as wind also blows at night.

 

I apologize for so many different posts. My power went out (go figure) while I was writing a previous post. I lost my train of thought. So, I am now going back through to address various different concerns.

 

Anyway, I agree completely, in having a solar array set up to include a wind turbine. (Many controllers have dual input for wind and solar, as well.) Living near the coast, I always notice some sort of breeze. if I can get a turbine with a low cut in wind speed, perhaps 2.0 m2 to 3.0 m2 ?.

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Bill H

Thank you Paul.  I've also been interested in this topic for quite some time.  I don't know about Cambodia, but in the States,the best buy for high amperage batteries are either 6v golf cart batteries, or the 2v battery cells if you have very, very deep pockets.  If your aim is to run the house, perhaps you should look at using 24v or 32v battery banks.  You can glean more amps AC this way.  Of course, the down side is you need to have more batteries to achieve larger amp storage.  Everything is a trade off in this area for sure.

 

This link is to a very good source of everything off grid.  Located in Arizona, but these guys really know their stuff and there is a wealth of information on their site.

 

http://www.solar-electric.com/

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miles-high

Sorry for my ignorance but I never thought of having batteries for emergency power. I would appreciate it if you would enlighten me as to the advantages of having the standby battery bank.

 

I always had a standby generator which helped us during the 4-day US North East Blackout of 2003 – I don’t know if the battery bank would last that long… Using the battery bank would be quieter and cleaner but I suppose you would need an inverter, while you get AC 110/240v from a genset (and probably much cheaper as a small genset can be had from PHP5,000 or so)…

 

:unknw:



 

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Bill H

Sorry for my ignorance but I never thought of having batteries for emergency power. I would appreciate it if you would enlighten me as to the advantages of having the standby battery bank.

 

I always had a standby generator which helped us during the 4-day US North East Blackout of 2003 – I don’t know if the battery bank would last that long… Using the battery bank would be quieter and cleaner but I suppose you would need an inverter, while you get AC 110/240v from a genset (and probably much cheaper as a small genset can be had from PHP5,000 or so)…

 

:unknw:

 

That's kind of an apples and oranges question.  The battery bank acts as a buffer to stand between your power input and your usage.  A battery bank allows you to run a smaller generator or use the power company until there is a failure.  Your battery bank can supply all your electric needs for hours or days without recharging, depending on your usage, the battery bank is always on in most configurations.  A generator is on or off.  When it's off there is no power.  When it's on there is, but you are paying for the fuel to keep it on.  Because there is no bank, you (in theory) would need a larger generator to meet your power needs.  That 5,500p generator you spoke of, will only produce a small amount of power and certainly not nearly enough to power an entire house in most cases.  Further, the battery bank can also be charged by solar cells and wind generators, so you don't have to run a diesel generator at all (depending again on the size of the system and the amount of power you use.)

 

There is of course a cost.  Battery banks are not cheap, nor are solar panels or wind generators.  So in the end, it all depends on whether a source of uninterpretable power is important to you and how much you're willing to pay to have it.

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Paul

I would appreciate it if you would enlighten me as to the advantages of having the standby battery bank.

 

In my case, I currently live in an apartment building that does not have an electric generator. (It should, but that is another story.)

 

Furthermore, there are some whinging, whiny-assed foreigners who live here, that only wish to complain, bitch, and moan to the owners whenever they can - about anything. So, in my case, a noisy generator is not really an option.

 

Unfortunately, adding solar panels to the buidling, after doing some in depth research, is not a good option for me, either. (I am on the ground floor of a three story building with a covered roof deck, which would mean I would need to run at least 15 to 18 meters of cable from the roof to my controller, batteries, and inverter. This would result in a serious voltage drop. (Going from the panels to the controller is DC voltage.)

 

In the same space I could install a generator, I could place a rack holding a bank of batteries, in a series-parallel configuration, and a voltage inverter large enough to run everything I need. They are similar to the ones on this page. This way, I will have a very quiet, alternative power supply.

 

So, the advantages are:

 

1. No fuel and oil expense.

2. Low maintenance. Installing a battery monitor will help significantly, here.

3. No noise pollution.

4. Only rack space is needed for the batteries and inverter.

5. The load can be connected to the inverter 24 / 7 / 365.

 

--------------

 

You mentioned having a generator to run for several days during extended power cuts. If you are in a home or other property where you can install solar or wind power, you can have a system that will constantly recharge itself, indefinitely.

 

Try not to allow the batteries to be discharged more than 50% before recharging, even though they are deep cycle batteries. Following this rule will extend the life of your batteries.

Edited by Paul
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